Impact of reduced rates of tiafenacil at vegetative growth stages on rice growth and yield

Abstract

Tiafenacil is a new nonselective protoporphyrinogen IX oxidase–inhibiting herbicide with both grass and broadleaf activity labeled for preplant application to corn, cotton, soybean, and wheat. Early season rice emergence and growth often coincide in the mid-southern United States with applications of preplant herbicides to cotton and soybean, thereby increasing the opportunity for off-target herbicide movement from adjacent fields. Field studies were conducted to identify any deleterious effects of reduced rates of tiafenacil (12.5% to 0.4% of the lowest labeled application rate of 24.64 g ai ha⁻¹) applied to 1- or 3-leaf rice. Visual injury 1 wk after treatment (WAT) for the 1- and 3-leaf growth stages ranged from 50% to 7% and 20% to 2%, respectively, whereas at 2 WAT these respective ranges were 13% to 2%, and no injury was observed. Tiafenacil applied at those rates had no negative season-long effect because observed early season injury was not manifested as a reduction in rice height 2 WAT or rough rice yield. Application of tiafenacil to crops directly adjacent to rice in its early vegetative stages of growth should be avoided because visual injury will occur. When off-target movement does occur, however, the affected rice should be expected to fully recover with no effect on growth or yield, assuming adequate growing conditions and agronomic/pest management are provided.

Introduction

By 2022, approximately 87% of all cropland acres in the United States was reported to be under some form of a conservation tillage production system, defined as tillage being reduced for at least one crop in a given field (Creech 2022). Of this conservation tillage system percentage, continuous no-till accounted for one-third of the hectares. Use of conservation tillage in crop production can lead to a potential reduction of 2,888 million liters in diesel equivalents per year, a reduction of 7.7 billion kg in associated emissions (Creech 2022). Realized benefits of conservation tillage systems can include improved soil health, decreased erosion, maximized water infiltration, improvement in nutrient cycling, and a build-up in organic matter (Creech 2022; Farmaha et al. 2021; Lal 2015).

Conservation tillage systems rely greatly on herbicides for effective preplant weed management. Numerous herbicides or combinations of herbicides are currently labeled and recommended for preplant or “burndown” control of many common and troublesome winter weed species encountered in corn, cotton, and soybean production fields (Anonymous 2023; Barber et al. 2024; Bond et al. 2024; McNeal et al. 2024; Stephenson et al. 2023). Weed resistance issues and difficult-to-control species have necessitated the identification of novel strategies and herbicides for continued successful preplant weed management in these production systems (Flessner and Pittman 2019; Johanning et al. 2016; Vollmer et al. 2019; Westerveld et al. 2021a, 2021b; Zimmer et al. 2018).

Tiafenacil, a new protoporphyrinogen IX oxidase (PPO)–inhibiting herbicide developed by FarmHannong Co., Ltd. (Seoul, Korea), exhibits nonselective contact activity on both weed and crop species (Anonymous 2023; Park et al. 2018). PPO-inhibiting herbicides halt the production of protoporphyrin IX (PPIX) from protoporphyrinogen IX, eventually preventing chlorophyll and heme biosynthesis. The increase in PPIX in the cytoplasm results in increases in singlet oxygen, which leads to lipid peroxidation, cell membrane destruction, and, ultimately, plant death (Shaner 2014). Tiafenacil is registered in the United States for preplant application to corn, cotton, soybean, and wheat, and for defoliation of cotton (Adams et al. 2022; Anonymous 2023). Limited published research on tiafenacil has focused on weed management. Tiafenacil at 74 g ai ha⁻¹ applied with varying volumes of urea ammonium nitrate as a carrier provided 85%, 81%, 92%, and 90% control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], common lambsquarters (Chenopodium album L.), kochia [Bassia scoparia (L.) A.J. Scott], and redroot pigweed (Amaranthus retroflexus L.), respectively, 1 WAT (Mookodi et al. 2023). Tiafenacil applied at 50 g ha⁻¹ alone resulted in 82% control of glyphosate-resistant downy brome (Bromus tectorum L.) (Geddes and Pittman 2023) 7 d after treatment (DAT), whereas the same rate co-applied with metribuzin at 400 g ha⁻¹ resulted in 88% control of glyphosate-resistant horseweed (Erigeron canadensis L.) (Westerveld et al. 2021b).

Rice was planted on more than 1 million ha in the United States in 2023 (USDA-NASS 2023). Rice emergence and early season growth often coincide with preplant herbicides applied in preparation for later planting of soybean or cotton and often occur in adjacent fields, thereby increasing the opportunity for off-target herbicide movement. Drift or off-target movement was previously identified by survey respondents from two separate states as the biggest herbicide application challenge they face (Butts et al. 2021; Virk and Prostko 2022). Additionally, severe crop injury from off-target herbicide movement is possible 60 m downwind from both ground and aerial applications, which can negatively affect yield, environmental stewardship, and other beneficial species (Butts et al. 2022). Consequently, it is imperative to understand the implications for crop growth and development if the crop were to be exposed to an herbicide drift event.

Serious deleterious effects of simulated off-target movement of selective and nonselective herbicides to rice at various growth stages have been demonstrated (Bond et al. 2006; Ellis et al. 2003). Rice growth stage at time of herbicide exposure has also been shown to result in differential sensitivity to herbicides labeled for use on rice. Patterson et al. (2023) reported that drill-seeded rice was more tolerant to application of benzobicyclon at the 4-leaf or tillering growth stages than early growth stages. Lawrence et al. (2021) investigated the effects of foliar application of sublethal rates of paraquat, a nonselective contact herbicide similar to tiafenacil, and fomesafen, a PPO-inhibiting herbicide like tiafenacil, to rice at the spike through panicle differentiation growth stages. Fomesafen injury 3 DAT exceeded 11% only with a preflood application. By 4 WAT, injury from fomesafen at any application timing ranged from just 2% to 5%. At 2 WAT, rice height was 95% of that of the nontreated control with fomesafen applied at the spike to 1-leaf growth stages; however, height ranged from 98% to 103% of the nontreated control for other timings. Rough rice yield was reduced with fomesafen applied later than the 2- to 3-leaf growth stage. Paraquat application injured rice by 37% to 47% regardless of application timing (Lawrence et al. 2021). Spike to 1-leaf and 2- to 3-leaf timings still exhibited 45% and 52% paraquat injury, respectively, at 4 WAT. Rice height 2 WAT was more negatively affected with exposure to paraquat prior to flooding compared with after flooding. Total and whole milled rice yield was not affected by paraquat applied at the spike to 1-leaf and 2- to 3-leaf timings compared with nontreated plants.

To our knowledge, no published information exists on the effects of tiafenacil on rice growth and yield following foliar application at sublethal rates that may be encountered in off-target movement events. Therefore, the objective of this research was to determine the effects on rice of foliar applications of tiafenacil as affected by growth stage at time of application.

Materials and Methods

Field experiments were conducted in 2022 at the Louisiana State University AgCenter Northeast Research Station near St. Joseph, LA (31.9184°N, 91.2335°W), the University of Arkansas System Division of Agriculture Lonoke Extension Center in Lonoke, AR (34.7843°N, 91.9001°W), and the Mississippi State University Delta Research and Extension Center in Stoneville, MS (33.4240ºN, 90.9151ºW) to determine the effects of reduced rates of tiafenacil (Reviton; HELM Agro US, Inc., Tampa, FL) applied at differing growth stages on rice growth and yield. Experiments were conducted in a randomized complete block design with treatments replicated three or four times. Treatments were applied via compressed air or CO₂-pressurized backpack sprayer at 140 L ha⁻¹. Treatments included a factorial arrangement of tiafenacil applied at 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× rates applied to one- or 3-leaf rice. The 1× rate basis for reduced rate calculation was 24.64 g ha⁻¹. The tiafenacil label (Anonymous 2023) allows single application rates from 24.64 to 75.04 g ha⁻¹, however, previous unpublished research has indicated that the lower rate in combination with glyphosate provides adequate cost-effective control of most common winter weed species prior to planting (DKM, personal observation). Methylated seed oil was added at 1% vol/vol to all treatments per label recommendations to maximize weed control (Anonymous 2023). A comparison 1% methylated seed oil–alone treatment was included but resulted in no effects on parameters measured in comparison to the 0× rate and was therefore excluded from statistical analysis. Tiafenacil at designated rates was applied to 1- or 3-leaf rice cultivar ‘PVL02’ near St. Joseph on May 26 or June 1, cultivar ‘Full Page RT 7521’ on May 18 or June 13 in Lonoke, and cultivar ‘CLL16’ on May 20 or June 9 in Stoneville. These timings were selected as being the most likely to exist when burndown of cotton and soybean ground normally occur in the mid-south (authors’ personal observations).

Plots were maintained weed-free at St. Joseph with a preemergence application of clomazone (Command 3 ME; FMC, Philadelphia, PA) at 656 g ha⁻¹ plus saflufenacil (Sharpen; BASF Corporation, Research Triangle Park, NC) at 50 g ha⁻¹; quizalofop p-ethyl (Provisia; BASF) at 119 g ha⁻¹, applied at the 2- to 3-leaf stage; halosulfuron plus prosulfuron (Gambit; Gowan, Yuma, AZ) at 83 g ha⁻¹, applied at the 3- to 4-leaf stage; and cyhalofop (Clincher; Corteva AgriScience, Indianapolis, IN) at 417 g ha⁻¹, applied at postflood. At Lonoke, plots were maintained weed free with a preemergence application of imazethapyr (Preface; ADAMA, Raleigh, NC) at 105 g ha⁻¹; and imazethapyr (Preface) at 105 g ha⁻¹ plus halosulfuron plus prosulfuron (Gambit) at 111 g ha⁻¹, applied at the 4-leaf stage. At Stoneville, plots were maintained weed free with a preemergence application of clomazone (Command 3 ME) at 559 g ha⁻¹; imazethapyr (Newpath; BASF) at 105 g ha⁻¹ plus quiclorac (Facet-L; BASF) at 420 g ha⁻¹, applied at the 2-to 3-leaf stage; and imazethapyr (Newpath) at 105 g ha⁻¹ plus halosulfuron (Permit; Gowan) at 39 g ha⁻¹, applied at the 4-leaf to 1-tiller stage.

Statistical Analysis

Parameter measurements included visual injury on a scale of 0 = no injury and 100 = plant death 1 and 3 WAT, plant height at 3 WAT for the 1-leaf timing and 2 WAT for the 3-leaf timing, and rough rice yield. The linear model (Equation 1) was fit to data as follows:

([1]) $$y = \beta_{0}+ \beta_{1}x + \varepsilon $$

where y represents the response variable of interest (visual injury, plant height; or rough rice yield); x represents the rate of tiafenacil (g ai ha⁻¹); β ₁ is the slope, the amount by which the response variable changes when the tiafenacil rate increases by one unit; β ₀ is the intercept, the value of the response variable when the tiafenacil rate = 0; and ε is the residual. The lm() function of the stats package was used to fit all linear models using R software (v. 4.3.3; R Core Team 2024). Data were analyzed by location and model parameters (slopes and intercepts) compared (Ritz et al. 2015) with no statistical differences detected between parameters of locations for herbicide rates applied at the same leaf stage (data not shown). Therefore, data were pooled across locations for curve fitting for a given application stage. In contrast, due to differences observed during application at 1-leaf and 3-leaf stages, data were analyzed separately for those stages. Model assumptions of linearity, homoscedasticity, independence, and normality were checked in each case.

Results and Discussion

Rice Injury

Visual rice injury was characterized by necrotic speckling of leaves contacted at time of application. When applied at the 1-leaf growth stage, rice was injured by 50% at the highest tiafenacil rate applied (1/8×), with each successive rate reduction resulting in 27%, 17%, 11%, 8%, and 7% injury 1 WAT (Figure 1). Exposure at the 3-leaf growth stage resulted in 20%, 11%, 10%, 4%, 3%, and 2% injury at those same rates (Figure 1). By 3 WAT, when applied at the 1-leaf growth stage, rice injury was 13% at the highest tiafenacil rate applied (1/8×), with each successive rate reduction resulting in 7%, 4%, 3%, 2%, and 2% injury (Figure 2). Application at the 3-leaf growth stage resulted in no visual rice injury 3 WAT (Figure 2). Lawrence et al. 2021 reported lower levels of injury than the current study, with injury no greater than 11% and 10% for the PPO-inhibiting herbicide fomesafen applied at 39 g ha⁻¹ to rice at the spike to 1-leaf and 2- to 3-leaf growth stages at 1 and 2 WAT, respectively. Differences in rice response may be due to natural sensitivity to the herbicides because tiafenacil exhibits effective activity on grass species, whereas fomesafen exhibits activity primarily against broadleaf weeds (Anonymous 2019, 2023). By 4 WAT, injury was no greater than 5%, which was similar to decreasing injury that occurred with time in the current study. Paraquat applied at 10% of the labeled rate injured spike to 1-leaf and 2- to 3-leaf rice by 44% to 47% at 1 and 2 WAT, respectively (Lawrence et al. 2021). At 4 WAT, injury at these timings was still 45% and 42%, respectively, indicating slower recovery from an early season application of paraquat than from tiafenacil.

Figure 1. Visual rice injury 1 wk after treatment (WAT) as affected by tiafenacil applied at 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× of a 24.64 g ai⁻¹ ha use rate applied to 1- or 3-leaf rice for data pooled across locations at St. Joseph, LA, Lonoke, AR, and Stoneville, MS, in 2022.

Figure 2. Visual rice injury 3 wk after treatment (WAT) as affected by tiafenacil applied at 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× of a 24.64 g ai ha⁻¹ use rate applied to 1- or 3-leaf rice for data pooled across locations at St. Joseph, LA, Lonoke, AR, and Stoneville, MS, in 2022.

Rice Height

Statistical analysis with respect to rice height indicated no negative effect of tiafenacil applied at either timing (Table 1). Rice height in the absence of tiafenacil averaged 26 and 35 cm at the 1- and 3-leaf growth stages, respectively (Figure 3). Height following tiafenacil exposure ranged from 25 to 26 cm and 37 to 35 cm at these respective growth stages. Lawrence et al. (2021) reported that rice height was 95% of the nontreated control at 2 WAT with fomesafen applied at the spike to 1-leaf growth stages. However, height ranged from 98% to 103% of that of the nontreated control for 2- to 3-leaf rice through panicle differentiation timings. Rice height 2 WAT was more negatively affected with exposure to paraquat prior to flooding (spike to 1-leaf through mid-postemergence timings) compared with post-flood (Lawrence et al. 2021). In the current study, at the 2 WAT assessment timing, rice was exhibiting much greater levels of injury than it did when tiafenacil was applied, which may explain the differences in height.

Table 1. Linear regression parameters for rice visual injury 1 and 3 wk after treatment, height, and rough rice yield following applications of tiafenacil. ^{a, b, c}

^a Abbreviation: WAT, weeks after treatment.

^b Tiafenacil was applied at rates of 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× of a 24.64 g ai ha⁻¹ use rate applied to 1- or 3-leaf rice. Data from 2022 were pooled across locations from St. Joseph, LA, Lonoke, AR, and Stoneville, MS.

^c Asterisks (***) indicate parameter significance.

Figure 3. Rice height 3 wk after treatment (WAT) for the 1-leaf timing and 2 WAT for the 3-leaf timing as affected by tiafenacil applied at 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× of a 24.64 g ai⁻¹ ha use rate applied to 1- or 3-leaf rice for data pooled across locations at St. Joseph, LA, and Lonoke, AR, in 2022.

Rice Yield

Similar to height, early season visual injury was not manifested in rough rice yield reduction following tiafenacil exposure (Table 1). Nontreated rice yield averaged 6,926 and 6,913 kg ha⁻¹ for the early and late growth stage timings, respectively (Figure 4). Yield following tiafenacil exposure ranged from 7,223 to 6,935 and 7,248 to 6,923 kg ha⁻¹ at these respective growth stages. Our results are similar to those reported by Lawrence et al. (2021) when early season rice visual injury observed with fomesafen and paraquat was not reflected in rough rice yield reduction at the spike to 1-leaf and 2- to 3- leaf exposure timings.

Figure 4. Rough rice yield as affected by tiafenacil applied at 0×, 1/8×, 1/16×, 1/32×, 1/64×, 1/128×, and 1/256× of a 24.64 g ai⁻¹ ha use rate applied to 1- or 3-leaf rice for data pooled across locations at St. Joseph, LA, Lonoke, AR, and Stoneville, MS, in 2022.

Practical Implications

Visual rice injury levels 1 WAT were greater at the 1-leaf growth stage than the 3-leaf stage when tiafenacil was applied at rates that ranged from 12.5% to 0.4% of the lower end of the labeled rate range (24.64 g ha⁻¹). Early season injury was evident quickly after application but lessened over time and was not manifested in height or rough rice yield reduction. The season-long response of rice to off-target application of tiafenacil applied at rates evaluated here would be similar to that of fomesafen, although tiafenacil exhibits effective activity on grass species, whereas tiafenacil exhibits primarily broadleaf activity (Anonymous 2019, 2023). Applying tiafenacil directly adjacent to rice in early vegetative stages of growth should be avoided because visual injury will occur. However, based on the current results and previous research with other PPO-inhibiting herbicides (Lawrence et al. 2021), tiafenacil appears to be a safer option for nonselective burndown weed control than paraquat applied to cotton or soybean fields near emerged rice before it reaches the 4-leaf growth stage. When off-target tiafenacil movement does occur, injured rice should be expected to fully recover with no impact on growth and rough rice yield (which is similar to the response to fomesafen and paraquat in previous studies), assuming adequate growing conditions and agronomic/pest management are provided.